Since a spacecraft like a space shuttle circling around the earth in the space has an interior thereof with balanced gravity and centrifugal force, a micro gravity environment which is nearly a gravity free environment is arranged. Under such a micro gravity environment, convection may not occur and sedimentation caused due to density difference can be avoided. Further, since such a micro gravity environment eliminates the necessity of using a container, optimum single crystal growth having no lattice defect has been expected. According to “Science, vol.270. p.1921-1922, 1995 entitled Search for Better Crystals Explores Inner, Outer Space”, European researchers have tried crystal growth of protein by utilizing space shuttle of United States, and they have succeeded in achieving optimum protein crystal growth which is almost impossible in the earth.
However, such a trial using the micro gravity environment in the space costs high and suffers drawbacks in terms of time and space. An approach of using a magnetic force has been proposed to overcome such drawbacks.
It is common knowledge that a magnet attracts materials like iron. Since ferromagnetic material like iron has a high magnetic susceptibility, it may be influenced by even a weak magnetic field. In addition, since the magnetic susceptibility has positive sign, the ferromagnetic material is attracted by a common permanent magnet.
Materials but the ferromagnetic material are classified into paramagnetic material and diamagnetic material. Such materials are free from the influence of common magnet, since they have extremely low magnetic susceptibility. However, paramagnetic and diamagnetic materials are influenced by a superconducting magnetic which generates strong magnetic field. Paramagnetic material is attracted by the magnetic because it has a magnetic susceptibility with positive sign, while diamagnetic material is repulsive against the magnet because it has a magnetic susceptibility with negative sign.
For convenience of explanation on the virtual micro gravity environment where magnetic force is utilized, a basic formula of the gravity and magnetic force which can be found in a gravitational field and magnetic field will be given. Each formula has physical quantity expressed in SI unit. All materials on the earth are under influence of gravity which acts downwardly on the material. The gravity per unit volume acting on the object with the density of p [kg/m3] can be expressed as follows.
[Formula 1]FG=ρ·g[N/m3]  (1) 
Wherein, g is acceleration of gravity and has a value of approximately 9.8 [m/s2] with a little difference according to the position on the earth. According to “Mcgraw-Hill Encyclopedia of Science and Technology, vol.10, Mcgraw-Hill, New York, p.314-316, 1992”, magnetic force per unit volume acting on the magnetic material positioned in the magnetic field can be expressed as follows.[Formula 2]                              F          M                =                              1            2                    ⁢                      χ                          μ              2                                ⁢          ▽          ⁢                                           ⁢                                    B              2                        ⁡                          [                              N                ⁢                                  /                                ⁢                                  m                  3                                            ]                                                          (        2        )            
Wherein, χ is volumetric magnetic susceptibility of magnetic material, μ is permanently, and B [T] is magnetic flux density. The magnetic force in case of gradient magnetic field where the magnetic flux density B changes only in z-axis direction, can be expressed as follows.[Formula 3]                              F          M                =                              χ                          μ              2                                ⁢                                    Bz              ⁡                              (                                  dBz                  ⁢                                      /                                    ⁢                  dz                                )                                      ⁡                          [                              N                ⁢                                  /                                ⁢                                  m                  3                                            ]                                                          (        3        )            
Therefore, in case where the magnetic force FM of Formula 3 has a size same as that of the gravitating force of Formula 1 and the magnetic force FM acts upwardly, the magnetic material disposed in the magnetic field is in a substantially gravity free environment. The gravity acting on the material can be controlled by utilizing the magnetic force FM since the magnetic force FM acts similarly with the centrifugal force.
FIG. 1 is a schematic perspective view and FIG. 2 is a sectional view illustrating a conventional apparatus for control of gravity acting on materials through the use of a superconducting magnet. A gravity control apparatus 10 include a hollow cylindrical low temperature container 11 with a hollow cylindrical bobbin 13 mounted thereto, wherein the bobbin is wound with a coil. When the coil is applied with electric power, a material 20 positioned in a hollow portion 15 of the low temperature container 11 is under gravity free or low gravity condition. The low temperature container 11 is generally made up of an aluminum or stainless steel, and the bobbin 13 is made up of FRP or aluminum. When the coil is applied with electric power and the magnetic force FM acts upwardly, the material 20 disposed in the magnetic field is under the micro gravity environment where F=FG−FM. When the coil is applied with electric power and the magnetic force FM acts downwardly, the material 20 disposed in the magnetic field is under over gravity environment where F=FG+FM.
Japanese researchers have reported that they have made success in flotation of material by utilizing magnetic force, and crystal growth of protein and marking of new material by utilizing virtual micro gravity space formed by magnetic force. For instance, according to “Symposium on New Magnetic Science '97-Japan, p.181-188”, they have made success in flotation of water, NaCl, Bi and the like by using a superconducting magnet, and synthesizing of new material. According to “Journal of Crystal Growth, vol. 178, p.653-656, 1997 entitled Effect of a magnetic field gradient on the crystallization of hen lysozyme”, gravity controlled by magnetic force may exert influence on the crystal growth of protein. However, the magnet employed for the above-mentioned study is disadvantageous in that the magnetic force which is marked in solid line in FIG. 3 is produced from both ends of the magnet having gradient magnetic field which is marked in a dotted line, and thus-produced magnetic force has non-uniformity in spatial distribution. Therefore, it is impossible to form a spatially uniform micro gravity space. A need therefore continues to exist for a magnet which generates spatially uniform magnetic force so as to uniformly control the gravity acting on material. When such a magnet is developed, an activity in the new field of study where a virtual micro gravity environment is adopted will be increased.